Conventionally, there is known an active matrix-type liquid crystal display apparatus including TFTs (Thin Film Transistors) as switching elements. This liquid crystal display apparatus includes a liquid crystal panel composed of two insulating glass substrates facing each other. One glass substrate of the liquid crystal panel is provided with gate bus lines (scanning signal lines) and source bus lines (video signal lines) in a grid pattern, and is provided with TFTs near the respective intersections of the gate bus lines and the source bus lines. Each TFT is composed of a gate electrode connected to a gate bus line; a source electrode connected to a source bus line; and a drain electrode. The drain electrode is connected to a corresponding one of pixel electrodes which are arranged on the glass substrate in a matrix form to form an image. The other glass substrate of the liquid crystal panel is provided with an electrode (this electrode is called a “common electrode”, “counter electrode”, etc., but is hereinafter called a “common electrode”) for applying a voltage between the pixel electrodes and the common electrode through a liquid crystal layer. Then, based on a video signal which is received, when a gate electrode of each TFT receives an active scanning signal from a corresponding gate bus line, by a source electrode of the TFT from a corresponding source bus line, a voltage which is the difference between the potential of the video signal and a potential provided to the common electrode is applied to the liquid crystal layer. By this, the liquid crystal is driven and a desired image is displayed on a screen.
Meanwhile, liquid crystal has the property of deteriorating with continuous application of a direct voltage thereto. Hence, in a liquid crystal display apparatus, an alternating voltage is applied to a liquid crystal layer. This will be described with reference to FIGS. 12 and 13. FIG. 12 is a diagram schematically showing a cross section of a liquid crystal panel of an active matrix-type liquid crystal display apparatus. As shown in FIG. 12, the liquid crystal panel is composed of a TFT array substrate 15 and a color filter substrate 16 which face each other with a liquid crystal layer 13 sandwiched therebetween. The TFT array substrate 15 has pixel electrodes 12, etc., formed thereon and the color filter substrate 16 has a common electrode 14, etc., formed thereon. Application of an alternating voltage to the liquid crystal layer 13 is implemented by reversing the polarity of an applied voltage (to the liquid crystal layer 13) in a portion forming each individual pixel (hereinafter, referred to as a “pixel formation portion”) every frame period. Specifically, driving of the liquid crystal display apparatus is performed such that the polarity of a voltage between the common electrode and each source electrode is reversed every frame period. Note that one frame period is a period for displaying an image for one screen on a screen.
For a technique for implementing driving of the liquid crystal display apparatus such as that described above, a driving scheme called line reversal driving, for example, is known. FIG. 13 is a signal waveform diagram for a liquid crystal display apparatus which adopts line reversal driving as its driving scheme. In FIG. 13, reference character THk (TH1, TH2, TH3, . . . ) indicates a period for selecting a gate bus line of a kth row (a horizontal scanning period including a horizontal flyback period), and reference character TF indicates one frame period. For a video signal VS, as shown in FIG. 13, its polarity (a positive or negative polarity with reference to a potential indicated by reference character 90) is reversed every horizontal scanning period, and furthermore, its polarity is also reversed every frame period. Likewise, for a common electrode signal VCOM for providing a desired potential to the common electrode 14, too, its polarity (a positive or negative polarity with reference to the potential indicated by reference character 90) is reversed every horizontal scanning period, and furthermore, its polarity is also reversed every frame period. In addition, the video signal VS and the common electrode signal VCOM are shifted in phase by 180 degrees (one horizontal scanning period) from each other. By this, the polarity of a voltage applied to the liquid crystal layer 13 is reversed every horizontal scanning period, whereby alternating driving of the liquid crystal display apparatus is implemented.
Meanwhile, in recent years, a liquid crystal display apparatus such as that described above has been adopted as a main screen of an electronic device such as a mobile phone. As one of such liquid crystal display apparatuses, there is one called a QVGA (Quarter Video Graphics Array) type having a resolution of 320×240. In such a liquid crystal display apparatus, it is pointed out that glass substrates composing a liquid crystal panel vibrate due to alternating driving such as that described above and the vibration is sensed as an annoying sound. To prevent the occurrence of such an annoying sound caused by vibration (hereinafter, referred to as “sound emission”), for example, a damping material is stuck on the liquid crystal panel, thereby attenuating the vibration.
It is known that sound emission occurs when a frequency (hereinafter, referred to as a “common electrode potential reversal frequency”) representing how often reversal of the potential of the common electrode 14 occurs (the term “reversal” as used herein refers to a change from a lower potential to a higher potential with reference to a predetermined potential or a change from a higher potential to a lower potential with reference to a predetermined potential) is in a human-audible frequency band. In a QVGA-type liquid crystal display apparatus, the common electrode potential reversal frequency is on the order of 10 kHz which is in the human-audible frequency band, and thus, the above-described sound emission remarkably appears. In view of this, Japanese Patent Application Laid-Open No. 2008-40195 discloses a technique for suppressing sound emission by bringing the common electrode potential reversal frequency out of the human-audible frequency band by reversing a common electrode signal VCOM during a predetermined period of one horizontal scanning period as shown in FIG. 14. Note that, in the following, a period of each horizontal scanning period during which the common electrode signal VCOM is maintained at a constant level is referred to as an “active period” and a period of each horizontal scanning period during which the common electrode signal VCOM is reversed at predetermined intervals is referred to as a “non-active period”.